Java: Convert DKIM private key from RSA to DER for JavaMail

Question

I'm using DKIM for JavaMail to sign outgoing mail with DKIM.
My private DKIM key is generated with opendkim-genkey -s default -d example.com and looks like this:

-----BEGIN RSA PRIVATE KEY-----
ABCCXQ...[long string]...SdQaZw9
-----END RSA PRIVATE KEY-----

The DKIM for JavaMail library needs the private DKIM key in DER format as stated in their readme file:

DKIM for JavaMail needs the private key in DER format, you can transform a PEM key with openssl:

openssl pkcs8 -topk8 -nocrypt -in private.key.pem -out private.key.der -outform der

I am looking for a way to avoid having to use openssl to convert my key to DER format. Instead I would like to do the conversion in Java directly.
I have tried different suggestions (1, 2, 3) but nothing has worked so far.
DKIM for Java processes the DER file like this:

    File privKeyFile = new File(privkeyFilename);

    // read private key DER file
    DataInputStream dis = new DataInputStream(new FileInputStream(privKeyFile));
    byte[] privKeyBytes = new byte[(int) privKeyFile.length()];
    dis.read(privKeyBytes);
    dis.close();

    KeyFactory keyFactory = KeyFactory.getInstance("RSA");

    // decode private key
    PKCS8EncodedKeySpec privSpec = new PKCS8EncodedKeySpec(privKeyBytes);
    RSAPrivateKey privKey = (RSAPrivateKey) keyFactory.generatePrivate(privSpec);

So in the end what I need is the RSAPrivateKey.

How can I easily generate this RSAPrivateKey that DKIM for JavaMail requires from my RSA private key?

score 4 · Accepted Answer · answered May 18 '14 at 00:28

Your reference 3 (only) is correct; as it says your problem is not just converting PEM to DER (which as @Jim says is basically just base64 to binary) but converting PEM containing openssl "traditional" or "legacy" or "PKCS#1" format key data to DER containing PKCS#8 (and specifically PKCS#8 clear/unencrypted) format key data.

The http://juliusdavies.ca/commons-ssl/pkcs8.html pointed to by Alistair's answer looks like it might be a possibility, but I didn't examine in detail. Since PKCS#8 clear (PrivateKeyInfo) for RSA is just a simple ASN.1 wrapper around the PKCS#1, the following (kinda) quick and (very) dirty code provides a minimal solution. Alter the input-reading logic (and error handling) to taste and substitute an available base64 decoder.

    BufferedReader br = new BufferedReader (new FileReader (oldpem_file));
    StringBuilder b64 = null;
    String line;
    while( (line = br.readLine()) != null )
        if( line.equals("-----BEGIN RSA PRIVATE KEY-----") )
            b64 = new StringBuilder ();
        else if( line.equals("-----END RSA PRIVATE KEY-----" ) )
            break;
        else if( b64 != null ) b64.append(line);
    br.close();
    if( b64 == null || line == null ) 
        throw new Exception ("didn't find RSA PRIVATE KEY block in input");

    // b64 now contains the base64 "body" of the PEM-PKCS#1 file
    byte[] oldder = Base64.decode (b64.toString().toCharArray());

    // concatenate the mostly-fixed prefix plus the PKCS#1 data 
    final byte[] prefix = {0x30,(byte)0x82,0,0, 2,1,0, // SEQUENCE(lenTBD) and version INTEGER 
            0x30,0x0d, 6,9,0x2a,(byte)0x86,0x48,(byte)0x86,(byte)0xf7,0x0d,1,1,1, 5,0, // AlgID for rsaEncryption,NULL
            4,(byte)0x82,0,0 }; // OCTETSTRING(lenTBD) 
    byte[] newder = new byte [prefix.length + oldder.length];
    System.arraycopy (prefix,0, newder,0, prefix.length);
    System.arraycopy (oldder,0, newder,prefix.length, oldder.length);
    // and patch the (variable) lengths to be correct
    int len = oldder.length, loc = prefix.length-2; 
    newder[loc] = (byte)(len>>8); newder[loc+1] = (byte)len;
    len = newder.length-4; loc = 2;
    newder[loc] = (byte)(len>>8); newder[loc+1] = (byte)len;

    FileOutputStream fo = new FileOutputStream (newder_file);
    fo.write (newder); fo.close();
    System.out.println ("converted length " + newder.length);

Aside: I assume the ABCC in your posted data was redacted. Any valid and reasonable PKCS#1 (clear) RSA key must begin with bytes 0x30 0x82 x where x is from 2 to about 9; when converted to base64 this must begin with MIIC to MIIJ.

I answered too quickly without looking at the references. I didn't consider the difference between BEGIN PRIVATE KEY and BEGIN RSA PRIVATE KEY. — Jim Flood, May 19 '14 at 01:21

score 0 · Answer 2 · answered May 17 '14 at 18:44

The PEM format is just the DER bytes encoded in Base64. You can just read the PEM file as a text file, and take only the lines in between "-----BEGIN RSA PRIVATE KEY-----" and "-----END RSA PRIVATE KEY-----".

Comments are allowed in the PEM file, so you may need to skip additional lines from the beginning until you reach the "-----BEGIN RSA PRIVATE KEY-----".

Here is some code in Ruby that converts PEM to DER just by Base64-decoding the text. You can translate this into Java:

require 'base64'

# Read the PEM as text
pem = File.read('key.pem')

# Break into individual lines
all_lines = pem.split("\n")

# Discard the first and last lines
b64_lines = all_lines[1..-2]

# Write the output file
File.open('key.der', 'w') do |f|
    # For each line
    b64_lines.each do |line|
        # Write the Base64-decoded bytes
        f.write Base64.decode64(line)
    end
end

Here is openssl displaying an RSA key I just created:

$ openssl rsa -text -in key.pem
Private-Key: (2048 bit)
modulus:
    00:aa:8d:15:05:db:a3:fa:bb:dc:8d:4f:d5:53:56:
    d6:78:7f:41:6d:05:b9:92:a4:7c:28:b1:11:50:9e:
    77:67:04:e9:77:22:62:db:1f:43:bc:f2:5e:fc:f3:
    42:35:a2:01:9c:9c:c1:90:b7:2b:1c:c8:51:f2:27:
... (omitted)

Here is openssl displaying the DER file that the Ruby code wrote:

$ openssl rsa -text -inform DER -in key.der
Private-Key: (2048 bit)
modulus:
    00:aa:8d:15:05:db:a3:fa:bb:dc:8d:4f:d5:53:56:
    d6:78:7f:41:6d:05:b9:92:a4:7c:28:b1:11:50:9e:
    77:67:04:e9:77:22:62:db:1f:43:bc:f2:5e:fc:f3:
    42:35:a2:01:9c:9c:c1:90:b7:2b:1c:c8:51:f2:27:
...

This was the PEM (middle part omitted):

Java: Convert DKIM private key from RSA to DER for JavaMail

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